Self-tensioning tie down system

ABSTRACT

A self-tensioning tie down assembly and safety system for tightening the retaining straps securing cargo on a carrier. Retaining straps are connected to winches connected to the carrier. A ratchet system is used to tighten the winch. A two bar torque linkage has a ratchet arm connectable to the ratchet wheel of the winch via a socket and has a connector arm connected to an actuator. The actuator is connected to a pneumatic/electrical circuit and provides reciprocating linear motion thereby actuating the torque linkage. The torque linkage may be set in a tightening or loosening position. The required tension level of the retaining strap is pre-set. Pressure within the circuit drives the actuator thereby actuating the torque linkage which tightens the strap on the winch until the tension level is reached. Signals are sent to a display panel to indicate when a strap is loose.

FIELD OF THE INVENTION

The present invention relates to a self-tensioning tie down assembly andsafety system, and more particularly to an assembly for automaticallytensioning tie down straps about a winch and relaying the status of thetie down straps to an operator.

BACKGROUND OF THE INVENTION

As the earth's population continues to grow and the global market forproducts expands, safe transportation for goods becomes increasinglyimportant. Goods are often transported by a carrier having some form ofplatform upon which the goods are placed, such as, but not limited to, aflat bed trailer. Any movement of the goods could result in damage tothe goods and/or create a safety hazard, for example if items beganfalling off the trailer thereby causing a motor vehicle accident. Afurther safety hazard can arise as a result of cargo shiftingsufficiently to create an unbalanced load thereby adversely affectingthe vehicle performance. This can prove especially dangerous when theflat bed trailer is cornering as the cargo shift may have increased thepropensity for the trailer to rollover. The goods must therefore besecured in order to prevent any moving or shifting. Various tie downassemblies have been developed for this purpose.

The tie down straps are typically fastened and tensioned with the aid ofwinch having a manual ratchet mechanism. Such ratchet mechanisms areknown in the art, for example, as shown in U.S. Pat. Nos. 4,045,002 and5,853,164, and may be used for several fastening applications. Theinherent limitation with these systems is the lack of monitoring andappropriate automatic adjustment of the strap tension during transit.This problem is a very common concern for flatbed trailers carryingcargo for the transportation industry. Operators are left wondering ifthey have adequately secured their cargo before departure and especiallyduring transit, when the contents may shift slightly, adverselyimpacting on the tension of the strap and on the load distribution.Another weakness with the manual ratchet mechanism is the risk of injuryto the operator. To manually secure tie downs, operators have to exertlarge forces, which often lead to physical injury.

An automatic take-up winch is taught in U.S. Pat. No. 4,036,476 toDouce. The Douce device employs an actuating unit that advances thewinch by applying force on the teeth of the ratchet wheel, one tooth ata time. While the automatic take-up winch of the '476 patent overcomesthe deficiency of the strictly manual winches, it suffers from a numberof its own deficiencies. For example, because the Douce winch relies onthe direct application of force against a tooth of the ratchet wheel,the force required is very high. The device therefore requires a strongsource of pressure, which can be problematic given the small amount ofavailable space below the carrier. In addition, the Douce invention issusceptible to failure as a result of debris build-up on the ratchetwheel teeth since the actuating unit needs to directly engage eachtooth. The constant force being applied to the teeth also leads toincreased wear of the teeth such that over time they will requirereplacement. Furthermore, the entire actuating unit and winch is onepre-formed unit such that the actuating unit may not be used inassociation with the winches that are already mounted on a carrier.

There are also no systems in place to monitor the tension of theretaining straps holding the cargo of a carrier in place and providefeedback to the operator of the carrier. The current tie down winchesused on cargo carriers employ a passive mechanical system that does notmonitor the condition or effectiveness of the system.

Accordingly, it is an object of an embodiment of the present inventionto provide an automated tie down mechanism having a monitoring andactive response system.

It is a further object of an embodiment of the present invention toprovide an automated tie down mechanism that can be used with theexisting winches on a carrier.

It is a further object of an embodiment of the present invention toovercome the deficiencies noted above.

Other objects of the invention will be apparent from the descriptionthat follows.

SUMMARY OF THE INVENTION

The invention consists of an apparatus that provides autonomousmonitoring as well as an active response to loosening tie down straps.The system is designed such that it is easily integrated to the existingtie down winch/ratchet mechanism that is used extensively in the flatbedtransportation industry. The existing tie down winch can still beutilized in its original format when the present invention isdisengaged. This provides users with the option to independently adjustand control the method of use for each individual winch.

The invention mimics the torque driven motion that is developed with atraditional human powered torque bar. The invention reduces the requiredlength of the torque bar by increasing the input driving force. Theinvention consists of coupling a two bar torque linkage with theexisting ratchet wheel on a winch assembly. A pneumatically drivenactuator powers the two bar torque linkage. One of the members in thetwo bar linkage is a reversible drive ratchet. The actuator produceslinear reciprocating motion which is transmitted and converted by thedrive ratchet into the required torque action; a tightening torque inone direction and freewheel in the other. The generated torque isdeveloped on the existing ratchet wheel by means of a socket attached tothe two bar linkage. The ratchet wheel is connected to a spool aboutwhich the tie down strap is wrapped. The forced rotation of the ratchetwheel and spool results in the tightening of the tie down strap. In thisfashion, the ratchet wheel and spool are tightened one tooth at a timeuntil the strap is tight. If the system is not powered, the user canmanually employ the existing ratchet mechanism in its non-automatedformat if so desired.

The invention advances the existing ratchet mechanism one ratchet wheeltooth increment at a time. A pawl prevents the ratchet from uncoilingwhen the two bar linkage and actuator are retracting and re-setting forthe next advance motion. When the ratchet mechanism advances, anelectrical signal is sent to a display panel that informs the operatorthat the tie down strap is loose. The signal is conveyed to the operatorwith a flashing LED light for a pre-set time period until the looseningcondition subsides.

According to the preferred embodiment of the invention there is provideda self-tensioning tie down assembly for use in automatically tighteninga retaining element on a winch securing cargo on a carrier. The assemblycomprises a mounting bracket, an acuator, a linkage, and a power source.The mounting bracket connects to the carrier adjacent to the winch andthe actuator is mounted on the bracket. The actuator has an extended andretracted positions and, when powered by the power source, providesreciprocating linear motion between the two positions. The linkage has aportion connected to the actuator and another portion connectable to thewinch. The actuator and linkage are adapted so that when the actuator isactuating, it transmits a tightening torque through the linkage to thewinch.

In another aspect, the actuator comprises a piston in a cylinder. Thepiston has a piston rod that is connected to a portion of the linkage. Apair of magnetic sensors are positionable on the cylinder and areadapted to sense the position of the piston within the cylinder and tosend a corresponding signal to the power source.

In another aspect, the linkage comprises a connector arm connected tothe actuator and a torque arm connected to the winch. The torque arm hasan arm portion and a drive ratchet portion. The arm portion is connectedto the connector arm and the drive ratchet portion is connectable to asocket, which in turn is connectable to the winch. The drive ratchetportion is fitted within the arm portion and is rotatable therein. Thedirection of rotation of the ratchet portion is selectable.

In another aspect, the power source comprises a source of pressurisedair and a control panel for controlling the release of this air. Thepressurised air is contained within a reservoir that is fillable fromthe carrier.

In yet another aspect, the control panel comprises a pressure regulatorand valve to adjust and activate the pressurised air. The pressureregulator is adjustable and once set controls the tension of theretaining element.

In another aspect the assembly is adapted to operate automatically totighten the winch until the retaining element reaches a desired tension.

According to a further embodiment of the invention there is provided aself-tensioning tie down assembly for use with a winch for tensioning atie down securing cargo to a carrier. The assembly comprises a mountingbracket, an actuator, a power source, a control panel, and a linkage.The mounting bracket is adapted to be mounted to the carrier. Theactuator is mounted to the mounting bracket and provides a reciprocatingaction when actuated. The power source provides power to the assembly,with the power being distributed by the power distribution system. Thelinkage converts the action of the actuator into a torque that isapplied to the winch.

According to a further embodiment of the invention there is provided amonitoring system for use with a self-tensioning tie down assembly foruse in automatically tightening a retaining element on a winch securingcargo on a carrier. The monitoring system comprises a display panel anda sensor. The display panel is positionable adjacent to the driver ofthe carrier and the sensor is adapted to sense when the tie downassembly is acting to tighten the retaining elements and to activate anindicator on the display.

In another aspect, the monitoring system further comprises a secondindicator on the display, wherein the indicator is activated when thetie down assembly is receiving power.

The foregoing was intended as a broad summary only and of only some ofthe aspects of the invention. It was not intended to define the limitsor requirements of the invention. Other aspects of the invention will beappreciated by reference to the detailed description of the preferredembodiment and to the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings and wherein:

FIG. 1 is a perspective view of a portion of a flat bed trailer equippedwith a preferred embodiment of a self-tensioning tie down assemblyaccording to the invention;

FIG. 2 is a front perspective view of the preferred embodiment of aself-tensioning tie down assembly according to the invention;

FIG. 3 is a rear perspective view of the assembly shown in FIG. 2;

FIG. 4 is a front plan view of the assembly shown in FIG. 2;

FIG. 5 is a sectional view taken along the line 5-5 shown in FIG. 4;

FIG. 6 is a perspective view of a display panel for use with aself-tensioning assembly according to the invention; and

FIG. 7 is a pneumatic circuit diagram of the system according to theinvention showing the connection between a plurality of self-tensioningtie down assemblies and a truck air line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A flatbed trailer 73 carrying cargo 71 restrained by tie down straps 70and equipped with a self-tensioning tie down assembly 2 and safetysystem according to the invention is shown in FIG. 1. The preferredembodiment of the self-tensioning tie down assembly 2 is shown in FIGS.2 through 5. Components of the assembly are described below, withidentical reference numbers used for identical parts.

Referring now to FIGS. 2, 3 and 7, the tie down assembly 2 has amounting bracket 16, an actuator 15, a two bar torque linkage 60, a pairof magnetic sensors 31, 32, a source of pressurised air 72 and apneumatic/electrical circuit 61. The circuit 61 comprises a 2-waycontrol valve 21, a pressure regulator 24, a pair of solenoids 19 on a4-way control valve 18, and an actuator 15 as shown in FIG. 3. The tiedown assembly is designed to be used in association with a winch 64mounted on a carrier. The retaining elements 70 used to retain the cargo71 as shown in FIG. 1 are tightened about the winch. Preferably theretaining elements are tie down straps, but could also take the form ofcables, ropes, chains, or the like. The assembly 2 provides automatictightening of the winch in order to tension the strap and can be used inconjunction with pre-existing winches already mounted on a carrier orcan be formed with a winch and the combined assembly and winch thenmounted in place on a carrier. The functioning of the components of theassembly 2 will be described in greater detail below.

The mounting bracket 16 is preferably in the form of a generally“C”-shaped channel, which is permanently welded to the underside frame62 of the flatbed trailer 73 as shown in FIGS. 2 and 3. The mountingbracket 16 serves as a rigid mount for the actuator 15 and the circuit61. The mounting bracket 16 has two slots 63 that allow for multiplemounting locations of the actuator 15 to accommodate a variety ofdiffering underside frame rails as found on different flat bed trailers.

The actuator 15 is attached to the mounting bracket 16 with four bolts29 for ease of removal should the actuator 15 require maintenance. Asshown in FIG. 5, the actuator 15 is preferably a piston 41 in a cylinder14, the piston having a piston rod 43, the actuator producing a doubleacting motion. The body of cylinder 14 is capped at either end by headwalls 47 and 49. Piston rod 43 passes through a sealed opening in headwall 47. The piston 41 divides the interior chamber of the cylinder intotwo chambers: a retract chamber 65 and an extend chamber 66, thechambers being located on either side of the piston 41. The piston 41and piston rod 43 are preferably driven by pressurised air in a linearreciprocating fashion as will be described in more detail below. It isalso contemplated that the actuator could take the form of any assemblyprovided it produces a double acting motion, for example an electricalor mechanical linear drive having a lead screw or gear.

Preferably, the pressurised air for use in the system is contained in areservoir 72 that is connected to a pressure distribution system in theform of conduit 74 as shown in FIG. 7. The 2-way control valve 21 ofeach self-tensioning tie down assembly 2 is connected to the pressuredistribution system 74. This 2-way control, valve 21 allows the user toactivate the assemblies 2 individually as required by the cargocharacteristics. The reservoir 72 draws a supply pressure from theexisting on-board pneumatic brake circuit of the flatbed vehicle. A maindirectional control valve 76 separates the reservoir 72 from the airbrake supply line 77 and air brake reservoir 75. When the control valve76 is opened, the reservoir 72 is filled with the same supply pressureas the brake lines 77. Once the reservoir 72 is full, this control valveis shut off so that no brake pressure is consumed by the various tiedown assemblies 2 in use. The user can activate the control valve 76,however, should the reservoir 72 require filling at a later stage.Preferably a local gauge is connected to the reservoir in order toprovide information on the status of the pressure level to the operatorin the cab.

Two magnetic sensors 31, 32 mounted on the external side of, and atopposite ends of, the cylinder 14 determine and control the range of thelinear motion of the actuator. Actual positioning of the magneticsensors can be varied depending on the length of reciprocating actionrequired. When the piston 41 is positioned in substantial radialalignment with, and adjacent to, one of the two magnetic sensors 31, 32,the respective magnetic sensor is activated. When a magnetic sensor isactivated, it sends an electrical signal to control valve 18. Based onthe electrical signal received, the control valve 18 directs thepressurised flow of air from the air source 72 into the retract chamber65 or extend chamber 66 of the actuator 15. More specifically, theelectrical signal activates the appropriate solenoid 19 on the controlvalve 18, which in turn causes the appropriate port 27, 28 to open.

When the retract magnetic sensor 32 is activated, a signal is sent tothe control valve 18, activating the appropriate solenoid and causingthe retract port 28 to open the flow of pressurized air to the retractchamber 65 of the actuator 15 while opening and exhausting the extendchamber 66 to the outside atmosphere. The pressure builds inside theretract chamber 65 of cylinder 14 forcing the piston 41 towards the headwall 49 and piston rod 43 to begin retracting into the cylinder 14. Thepiston 41 and piston rod 43 continue to retract until the extendmagnetic sensor 31 is triggered by the proximity of the piston 41. Themagnetic sensor 31 sends a corresponding electrical signal to thecontrol valve 18, thereby activating the corresponding solenoid. Thesolenoid causes the extend port 27 on the control valve 18 to open andexhaust the retract chamber 65 to the outside atmosphere while incominghigh pressure is directed towards the extend chamber 66. The highpressure air entering the extend chamber 66 forces the piston 41 towardsthe head wall 47. In this fashion the actuator 15 continues to cycle ina reciprocating linear motion between the two magnetic sensors 31, 32until an equilibrium state is reached. Conduits (not shown) such asrubber hoses or the like are used to connect the various components ofthe circuit. The aforementioned system is shown as a pneumatic circuitschematic in FIG. 7. It should be noted that any number of tie downassemblies 2 and their corresponding pneumatic/electrical circuits 61may be employed such that the system is not limited to the diagram shownherein.

The reciprocating linear motion of the actuator 15 is transmitted as anequivalent reciprocating rotational motion to the winch 64 through thetwo bar torque linkage 60. The two bar torque linkage has a connectorarm 11 and a reversible drive ratchet torque arm 17. The end of thepiston rod 43 is connected to the connector arm 11, which in turn isconnected to the drive ratchet torque arm 17. The connector arm 11 isconnected to the end of the piston rod 43 by way of a pin joint 12 orthe like. At one end the torque arm 17 is connected to the connector arm11 by way of a pin joint 12. At the other end the torque arm 17 isattached to a socket 9, which in turn is connected to a ratchet wheel 7having teeth 4 as shown in FIG. 2. Preferably, the socket 9 ispermanently welded to the center of the ratchet wheel 7. The torque arm17 is preferably a cut off standard drive/torque ratchet that snaps intothe socket 9.

The drive ratchet arm 17 can transfer torque in one direction whilefreewheeling in the other. The drive ratchet arm 17 is a reversiblemechanism that is capable of reversing the drive/freewheel directionthrough the reversing lever 10. The reversing lever has two positions: atightening and a loosening configuration. When the reversing lever 10 isoriented in a tightening configuration as shown in FIG. 2, the driveratchet arm 17 transfers a tightening torque to the socket 9 and ratchetwheel 7 when the actuator 15 is retracting. With the reversing lever inthe tightening orientation, the drive ratchet arm 17 freewheels when theactuator is extending. The opposite occurs when the reversing lever isin the loosening orientation. The reversibility of the system ensuresthat it may be connected with winches having either clockwise or counterclockwise tightening rotation and allows for the loosening of thewinches as discussed further below.

When the system is activated, the actuator 15 retracts and rotates thetorque arm 17 in a tightening direction. The torque arm in turn rotatesthe socket 9, which forces the ratchet wheel 7 to rotate with the torquearm 17. The arm 17 continues to rotate until the extend magnetic sensor31 is triggered. Concurrently a pawl 8 connected by pin 30 to theexisting winch 64, is engaged in the dedendum section 25 of a tooth 4 onthe ratchet wheel 7 as shown in FIG. 5. In this configuration the pawl 8resists the unloading motion of winch 64. Since the ratchet wheel 7 isheld in this position by pawl 8, the torque arm 17 is free to rotate(freewheel) in a loosening direction without unloading the stored torquein the winch 64. The actuator 15 extends to its fullest position as setby the positioning of the retract magnetic sensor 32 and resets itselffor the cycle to continue when the retract sensor 32 is once againtriggered.

The cycle continues until such time as an equilibrium state is reached.In this state the driving force from the actuator 15 and correspondingtorque developed on the winch 64 are equalized by the force andcorresponding torque developed by the resistance in the tie down strap70, which is wrapped about spool 50. This state of equilibrium resultsin no further motion in the system until such time as the input torqueis larger than the resistance torque. During transit, slack is oftendeveloped in the strap 70 that would cause the resistance torque to dropand the system to once again re-tighten the strap 70 to equalize thetorques. In this fashion the system is continuously applying atightening torque on the winch 64.

Should the operator wish to loosen the tie down strap, the system needsimply be depressurised and reversing lever 10 switched from tighteningto loosening. The tension in the strap will cause the ratchet assembly64 to loosen until pawl 8 restricts the unwinding motion. The user canmanually disengage pawl 8 and completely undo the strap 70.

The amount of applied torque can be adjusted to accommodate differentcargo. This adjustment is made by manual modification to a regulator 24that controls the amount of pressure input to the main actuator 15. Agauge 22 is connected to each regulator 24 so that the operator candetermine the desired pressure. Each tie down assembly 2 connected tothe system has its own regulator 24 such that the applied torque can beindividually set for each tie down assembly 2. This allows for the safetransport of various cargo and loading configurations on the sameflatbed.

The present invention acts to ensure that each strap will be tightenedto the pre-set tension whenever slack develops, this tension beingregulated by the pressure. The device is able to accommodate for changesin climate during transit; for example, an increase in temperaturecausing the strap to expand such that slack develops. The system of theinvention will sense this and tighten the strap to the pre-set tension.

Preferably, the system is adapted to monitor the activity of eachindividual tie down assembly 2 and its corresponding winch 64 andcommunicates that information to the user. A display panel 67 consistingof LED's reflects the activity of each tie down assembly 2 connected tothe system, as shown in FIG. 6. Preferably, the display panel isconnected to the tie down assembly, more specifically, the retractmagnetic sensor is connected to the LEDs. When the retract magneticsensor is activated (indicating the ratchet wheel is indexing to thenext tooth) a signal is sent to and converted to a flashing light. Agreen LED light 69 is illuminated for each individual tie down assembly2 that is powered on by the system. A red flashing LED 68 is illuminatedfor each individual tie down assembly 2 that is in motion indicating aloose strap.

The display panel is programmed to automatically shut off the red LED 68after a set period of time. Preferably, the set period of time will beslightly longer than the time required by the tie down assembly tocomplete one cycle of the piston from the retract sensor to the extendsensor and back to the retract sensor. As long as the retract magneticsensor continues to be activated as the piston cycles, the red LED 68will continue to flash. In this way, the red LED 68 will continue toflash until the loose condition subsides and the tie down assembly stopsindexing the ratchet wheel 7 of the winch 64. The display panel 67 ispreferably housed in the cab of the truck that is towing the trailer.The signals can be sent between the truck and trailer by means of ahard-wired connection or short distance wireless technology.

It is also contemplated that other display systems could be used. Forexample, the display could have one green light and one red light forthe entire system, thereby reducing the cost. While this does notprovide the added detail of the display system discussed above, it isexpected that once a problem has been identified to the driver, thedriver will stop to check the cargo.

It will be appreciated by those skilled in the art that the preferredand alternative embodiments have been described in some detail but thatcertain modifications may be practiced without departing from theprinciples of the invention.

1. A self-tensioning tie down assembly for use in automaticallytightening a retaining element on a winch securing cargo on a carriercomprising: a mounting bracket connected to said carrier adjacent tosaid winch; an actuator having an extended position and a retractedposition, said actuator being mounted to said bracket and when poweredproviding reciprocating linear motion between said extended andretracted positions; a linkage, a portion of said linkage beingconnected to said actuator and another portion of said linkage beingconnectable to said winch; a power source for actuating said actuator;wherein when said linkage is connected to said winch and when saidactuator is actuating, said actuator transmits a tightening torquethrough said linkage to said winch; and a display unit connected to acircuit, said circuit being mounted to said bracket, wherein saidcircuit sends a signal to said display unit to indicate when saidactuator is being actuated.
 2. A self-tensioning tie down assembly foruse in automatically tightening a retaining element on a winch securingcargo on a carrier comprising: a mounting bracket connected to saidcarrier adjacent to said winch; an actuator having an extended positionand a retracted position, said actuator comprising a piston in acylinder, said piston having a rod, said actuator being mounted to saidbracket and when powered providing reciprocating linear motion betweensaid extended and retracted positions; a linkage, a portion of saidlinkage being connected to the rod of said piston of said actuator andanother portion of said linkage being connectable to said winch; a powersource for actuating said actuator; wherein when said linkage isconnected to said winch and when said actuator is actuating, saidactuator transmits a tightening torque through said linkage to saidwinch, said actuator further comprising a pair of magnetic sensorspositioned on said cylinder, said sensors adapted to sense the positionof the piston within said cylinder and to send a corresponding signal tosaid power source.
 3. A self-tensioning tie down assembly for use inautomatically tightening a retaining element on a winch securing cargoon a carrier comprising: a mounting bracket connected to said carrieradjacent to said winch; an actuator having an extended and a retractedposition, said actuator comprising a piston in a cylinder, said pistonhaving a rod, said actuator being mounted to said bracket and whenpowered providing reciprocating linear motion between said extended andretracted positions; a linkage, comprising a connector arm connected tothe rod of said piston of said actuator, said linkage further comprisinga torque arm connectable to said winch, wherein said torque armcomprises an arm portion and a socket portion, said arm portionconnected to said connector arm and said socket portion connectable tosaid winch; a power source for actuating said actuator; and wherein whensaid linkage is connected to said winch and when said actuator isactuating, said actuator transmits a tightening torque through saidlinkage to said winch.
 4. The assembly of claim 3 wherein said socketportion is selectively rotatable relative to said arm portion.
 5. Theassembly of claim 4 wherein the direction of rotation of said socketportion relative to said arm portion being selectable.
 6. The assemblyof claim 5 wherein said power source comprises a source of pressurisedair supplied to said actuator, the assembly further comprising a controlpanel for controlling the actuation of said actuator by controlling theapplication and venting of said pressurised air supplied to saidactuator.
 7. The assembly of claim 6 wherein said control panelcomprises a pressure regulator and valve to adjust and activate saidpressurised air.
 8. The assembly of claim 7 wherein said pressureregulator is adjustable, to control the actuation of said actuator totransmit tightening torque through said linkage to said winch to producethe desired tension in said retaining element.
 9. The assembly of claim6 wherein said cylinder comprises a first chamber being positioned onone side of said piston and a second chamber being positioned on a sideof said piston opposite said first chamber, the size of said chambersbeing alterable by movement of said piston within said cylinder.
 10. Theassembly of claim 9 wherein said control panel alternates between firstapplying pressurised air into said first chamber and venting air fromsaid second chamber thereby forcing said piston towards said secondchamber and second applying pressurised air into said second chamber andventing air from said first chamber thereby forcing said piston towardssaid first chamber.
 11. The assembly of claim 10 wherein said actuatorfurther comprises a pair of magnetic sensors positioned at opposite endsof said cylinder, each said sensor adapted to sense when said piston isin proximity to said sensor.
 12. The assembly of claim 11 wherein saidsensors send a signal to said control panel when they sense said piston,said control panel upon receiving said signal from one of said sensorsacting to apply and vent pressurised air as necessary.